1. Field of the Invention
The present invention relates to an indexable insert which is attached to a tool body of a cutting tool and is used for cutting metal materials, etc.
2. Discussion of the Background
Hitherto, there has been known an indexable insert in which a chip breaker having a specified sectional configuration is formed on a rake face for the ejection of chips. In such an indexable insert, in order to improve the chip ejection efficiency, the slope of a wall surface 2 of the chip breaker, which rises from a rake face 1, has been set depending on the depth of the cut (infeed) as shown in FIGS. 26 and 27, for example. Specifically, an indexable insert having a cross-section shown in FIG. 26 is intended to improve the chip ejection efficiency in a low infeed range by setting an angle .alpha. of the slope of the breaker wall 2 so that the breaker wall rises steeply from an intersection 3 between rake face 1 and the breaker wall. Also, an indexable insert having a cross-section shown in FIG. 27 is intended to improve the chip ejection efficiency in a high infeed range by setting the angle .alpha. to be so small that the breaker wall 2 rises gently from the intersection 3.
These conventionally known indexable inserts, however, have disadvantages as follows. In the indexable insert shown in FIG. 26, chips are apt to stuff, so that cut resistance is increased in the high infeed range because the height from an edge 4 to a boss surface 5, i.e., the height h of the breaker wall 2, is too large. Also, in the indexable insert shown in FIG. 27, the breaking action to make chips curled is weak in the low infeed range because the slope of the breaker wall 2 is too gentle. Accordingly, chips tend to become more elongate and the chip ejection efficiency is reduced. Thus, each of the indexable inserts having the above-described constructions have a superior chip ejection ability under one cutting condition, i.e., in the low infeed range or the high infeed range, but suffers from a lowering of the chip ejection efficiency or an increase of the cut resistance under the other cutting condition.
Meanwhile, as shown in FIGS. 28 to 31, by way of example, an indexable insert having a chip breaker with a varied sectional configuration around the corner (nose portion) of a rake face has also been proposed. The tip shown in FIGS. 28 to 31 is constructed such that an upper surface of a tip body 11, which is formed as a flat plate which is substantially rhombic in plan, serves as a rake face 12, lateral surfaces of the tip body 11 each serve as a flank face 13, and an edge 14 is formed along the ridge defined at an intersection between the rake face 12 and each flank face 13, i.e., along the ridge defined by each side of the rhombic rake face 12. Further, at two of the four corners C, C of the rake face 12, where the edges 14 adjacent to each other intersect at an acute angle, a convex arc-shaped nose portion edge 15 is formed in a smooth continuous relationship with the edges 14 on both sides. The rake face 12 is recessed at a specified slope as it extends inwardly from the edge 14 and the nose portion edge 15. The edge 14 and the nose portion edge 15 thus have a positive rake angle .theta..
In an inner area of the rake face 12, a chip breaker 16 protrudes upwardly from the rake face 12 with gaps left from the edges 14 and the nose portion edges 15. The chip breaker 16 is formed, as shown in FIG. 28, such that it has a breaker wall surface 17 with a rhombic shape and a size smaller than the rake face 12 inwardly of the edges 14, in a plan view looking in a direction facing the rake face 12 with a uniform gap left from the edges 14. At the corners C, C adjacent to the nose portion edges 15, it extends in the diagonal direction connecting the corners C, C so as to terminate in positions close to the nose portion edges 15. Reference numeral 18 in FIG. 28 denotes an attachment hole in which is inserted a clamp screw or the like used for attaching the tip body 11 to a tool such as a bite.
In practical cutting with the above-mentioned indexable insert, when an infeed and/or a feed is relatively small and an area just around the nose portion edge 15 is used as occurs in finish cutting, the nose portion edge 15 produces chips which are thin and have a small width. These chips are quickly broken into pieces by the above-mentioned indexable insert because they strike against a fore end of the chip breaker 16 positioned close to the corresponding nose portion edge 15 immediately after being produced. On the other hand, when an infeed and/or a feed is relatively large and an area extending from the nose portion edge 15 to the edge 14 is used as occurs in rough cutting, the nose portion edge 15 produces chips which are thick and have a large width. These chips are broken into pieces by the above-mentioned indexable insert in such a manner that they are subject to resistance while sliding over the rake face 12 between the edge 14 and the chip breaker 16, and then strike against the chip breaker 16, whereby they are so bent and curled as to break into pieces. Accordingly, the above-mentioned indexable insert can achieve efficient chip ejection in general universal cutting ranging from rough cutting to ordinary finish cutting.
However, when an infeed and/or a feed is set to a smaller value than in the ordinary finish cutting as occurs in, e.g., superfinish cutting required in precision machining, the produced chips become harder to break into pieces because they are thinner, have a narrower width and tend to be more elongate. At the same time, the part of the nose portion edge 15 which is used for cutting and produces chips also becomes smaller. This results in a difficulty in causing the produced chips to surely strike against the fore end of the chip breaker 16 and break into pieces for ejection even with the universal cutting tip shown in FIGS. 28 to 31. When used in superfinish cutting, therefore, such a universal cutting tip must be modified to make the produced chips surely strike against the breaker wall surface 17 at the fore end of the chip breaker 16 such that the fore end of the chip breaker 16 is positioned closer to the nose portion edge 15, or that the chip breaker 16 has an increased height in a direction of thickness of the tip body 11, or that the breaker wall surface 17 at the fore end of the chip breaker 16 protrudes more steeply in its entirely.
These modified constructions however are not satisfactory for the following reasons. If the fore end of the chip breaker 16 is positioned too close to the nose portion edge 15, when the modified indexable insert is employed in general universal cutting, particularly in rough cutting during which thick and wide chips are produced, the chips strike against the chip breaker 16 before sliding a sufficient distance over the rake face 12, and a pocket space defined between the edge 14 or the nose portion edge 15 and the chip breaker 16 becomes small. As a result, the chips are apt to stuff and the chip ejection ability deteriorates which is not the desired result. Also, easier stuffing of the chips may cause the cut resistance to be so increased as to cause severe vibration, deterioration of finished surface accuracy and shortening of the tip life.
Further, if the fore end of the chip breaker 16 is positioned closer to the nose portion edge 15, the recessed inflecting point P where the breaker wall surface 17 at the fore end of the chip breaker 16 intersects the rake face 12 is also positioned closer to the nose portion edge 15. The closer position of the nose portion edge 15 to the recessed inflecting point P, however raises a fear that the tip body 11 may break at the recessed inflecting point P when the tip is used to cut hard materials, because stresses caused by the cutting load acting upon the nose portion edge 15 tend to be concentrated on the recessed inflecting point P. Further, such a possibility of the tip breakage increases with the rake angle .theta. having a larger value. In the above-mentioned indexable insert, therefore, it is impossible to improve the sharpness of the edges 14 and the nose portion edges 15 by setting the rake angle .theta. to a larger value. This means that a further increase in the cut resistance would result.
On the other hand, if the height of the chip breaker 16 is increased, or if the breaker wall surface 17 protrudes more steeply, the chip ejection ability also deteriorates in universal cutting because the produced chips are less smoothly ejected and are more apt to stuff. Additionally, if the breaker wall surface 17 protrudes more steeply, the cut resistance is further increased in universal cutting because thick and wide chips are produced and strike against the breaker wall surface 17.
Up to now, therefore, the tip designed for superfinish cutting, as described above, has had difficulty in covering the entire range of general universal cutting, and a wide range of cutting including general universal cutting and superfinish cutting must have been performed by respective dedicated tips prepared separately.